In order to enhance the recovery of subterranean fluids, such as oil and gas, it is sometimes desirable to orient the direction of the wellbore or borehole. In an oil producing formation or strata which has limited vertical depth and relatively greater horizontal extent with respect to the surface of the earth, a wellbore which extends horizontally through the oil producing formation may be more productive than one extending vertically. In order to create an inclined, highly deviated or horizontal borehole, it is necessary to steer the drilling bit at the end of the drill string from a generally vertical orientation to a lateral or horizontal orientation.
In steering a drill bit and drill string from a vertical orientation to a horizontal or other non-vertical orientation, it is necessary to deflect or side-track the drill bit from the generally vertical borehole to a drilling direction inclined to the wall of the borehole. This initial step is also known as “kicking off” or a kick-off of the drill bit and drill string. It is typical to first drill a vertical wellbore and then attempt to deflect the drill bit and drill string by some means thereby causing it to drill through the wall of the existing wellbore.
When drilling a deviated wellbore from a cased well, a section of the casing must first be cut and removed. Once the casing is removed, an opening is provided for the drill bit to pass through the casing into the cement surrounding the casing and then through the formation.
The development of techniques for drilling relatively high angle deviated wells from a generally vertical wellbore has provided several advantages in recovering oil and gas from subterranean formations. One or more deviated or generally horizontal wellbores may be drilled from single generally vertical wellbore to provide wellbores which: (a) reach irregular reservoirs without additional wells being drilled from the surface, (b) limit the invasion of unwanted formation fluids, (c) penetrate natural vertical fractures (d) improve production from various types of formations or reservoirs and (e) provide new conduits for hydrocarbons to flow to the surface.
The creation of multi-lateral wells from either new or existing wellbores usually involves some sort of sidetracking process that utilizes whipstocks and/or section mills to create an exit point in the casing to allow a drilling assembly to “kick-off” from the main wellbore. During such procedures, communication is often severed with the main wellbore below the point of kick-off, thus eliminating the use of the lower portion of the main wellbore for the continued production of hydrocarbons. Also, these multi-lateral construction procedures result in wellbore diameters that are the same size or smaller than the existing wellbore.
For multi-lateral well drilling, section mills are generally not used since they result in complete severing of the main wellbore, which makes re-locating the main wellbore below the casing exit point difficult, if not impossible. Nonetheless, section mills offer the potential to improve multi-lateral well juncture construction due to the greater available space for creating a sealed hydraulic juncture while maintaining full-bore accessibility.
Downhole excavation would facilitate construction of multi-lateral wells in several ways. First, downhole excavation would allow creation of a large downhole cavity while maintaining access to the existing wellbore below the excavation point. Second, downhole excavation would provide an alternative method for “kicking off” the lateral from the main well bore in multi-lateral well drilling. Third, downhole excavation would provide a large area to facilitate construction of a hydraulically sealed juncture that maintains full-bore accessibility. Fourth, downhole excavation would provide a large excavated location where multiple laterals could be kicked-off from the same location in the parent casing bore.
A significant amount of technology has been developed in the area of multi-lateral wells and section milling to enlarge boreholes. However, no examples are available demonstrating the use of section mills to create large downhole cavities for multi-lateral well construction and permit access to parent casing bore below the kick-off.
Accordingly, there is a need for a method and apparatus to provide an excavation suitable for a multi-lateral well while maintaining full bore access below the excavation. This invention satisfies that need.